The present invention relates generally to an electrophotographic printing machine, and more particularly concerns a method and apparatus for predicting the life of a photoconductive imaging member used in the printing machine.
Generally, the process of electrophotographic printing includes charging a photoconductive member to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive surface is exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive member while leaving behind an electrostatic latent image in the non-illuminated area. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic toner particles on the surface of the photoconductive member. The resulting visible toner image can be transferred to a suitable receiving member such as paper. This imaging process may be repeated many times with reusable photoconductive imaging members.
A flexible photoreceptor belt, one type of photoconductive imaging member, is typically multi-layered and has a substrate, a conductive layer, an optional hole blocking layer, an optional adhesive layer, a charge generating layer, a charge transport layer, and, in some embodiments, an anti-curl backing layer. High speed electrophotographic copiers and printers use flexible photoreceptor belts to produce high quality toner images. During extended cycling of the belts, a level of reduced life is encountered, which requires belt replacement in order to continue producing high quality toner images. As a result, photoreceptor characteristics that affect the image quality of toner output images as well as photoreceptor end of life, have been identified.
Photoreceptor characteristics that affect image quality and ultimately determine photoreceptor end of life include; charge acceptance when contacted with a given charge, dark decay in rested (first cycle) and fatigued state (steady state), the discharge or photo induced discharge characteristics (PIDC) which is the relationship between the potential remaining as a function of light intensity, the spectral response characteristics and the residual potential. As photoreceptors age, they undergo conditions known as cycle-up and cycle-down. Cycle-up (residual rise) is a phenomenon in which residual potential and/or background potential keeps increasing as a function of cycles, which generally leads to increased and unacceptable background density in copies of documents. Cycle-down is a phenomenon in which the dark development potential (potential corresponding to unexposed regions of the photoreceptor) keeps decreasing as a result of dark decay as a function of cycles, which generally leads to reduced image densities in the copies of documents.
Copiers and printers with cycling photoreceptor belts utilize many different methods of measuring photoreceptor characteristics. U.S. Pat. No. 3,898,001 to Hardenbrook et al. discloses a system of non-contact measurement of electrostatic change. U.S. patent application Ser. No. 07/636,045, filed on Dec. 28, 1990, entitled "Motionless Scanner", assigned to the same assignee as the present invention, describes a process for ascertaining the electrical discharge properties of a photoreceptor. These and other methods of measuring photoreceptor properties are used to diagnose a photoreceptor's ability to produce quality output images. These methods, however, do not predict when a photoreceptor will experience diminishing properties, they generally only provide feedback as to the present condition of the various characteristics that make up a photoreceptor.
U.S. patent application Ser. No. 07/636,034, filed on Dec. 28, 1990, entitled "Photoreceptor Assessment System", assigned to the same assignee as the present invention, describes a process for ascertaining the projected imaging life of a photoreceptor independent of machine (copier or printer) interactions. The process assesses virgin samples of photoreceptors from the output of a manufacturing line to determine the expected life of the photoreceptor to the point of failure due to unacceptable dark decay. Samples are cycled and charged at a constant voltage. Resulting dark decay after charging generally increases with cycling, but levels off at a crest value after a few cycles. The crest value and the associated number of imaging cycles are then used as a measure to evaluate the projected cycling life of the virgin sample.
Different copier and printer machines using varying sized photoreceptors that have been made from identical materials exhibit different life spans. Belts from different production runs have different life spans as well. As a result, a feedback control system constantly adjusts machine operating parameters to compensate for variations in photoreceptor electrical properties. When operating parameter adjustments to the machine can no longer compensate for degrading photoreceptor properties, the photoreceptor is typically replaced. In order to avoid unnecessary machine down-time due to a photoreceptor hard failure (when the machine is no longer able to compensate for degraded photoreceptor properties), photoreceptors are typically replaced when a plurality of soft faults warn the user of an impending hard failure. Soft faults typically are exceeded threshold values for photoreceptor properties that approach hard failure values.
U.S. Pat. No. 5,016,050 to Roehrs et al. discloses a control system that measures the dark decay of a photoreceptor belt to set a bias voltage on a shield of a dicorotron. As the belt ages, dark decay starts to increase and the bias on the shield increases to compensate for increasing dark decay. When the value of the bias on the shield of the dicorotron reaches a predetermined maximum level, the operator is warned by a photoreceptor near end of life message displayed on the user interface of the machine. Replacement of photoreceptors as a result of fault warnings in general tends to be premature. Unnecessarily shortening photoreceptor lives invariably increases the cost of machine maintenance. Predictive methods using soft faults generally are a function of a threshold value which is set from a population of machines. It is therefore desirable to predict the end of life of a photoreceptor more accurately in order to avoid unnecessary maintenance costs due to premature photoreceptor replacement.